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Gold-loaded Tin Dioxide Gas Sensing Materials: Mechanistic Insights and the Role of Gold Dispersion David Degler, Sven Rank, Sabrina Mueller, Hudson W. Pereira de Carvalho, Jan-Dierk Grunwaldt, Udo Weimar, and Nicolae Barsan ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00477 • Publication Date (Web): 28 Oct 2016 Downloaded from http://pubs.acs.org on October 31, 2016
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Gold-loaded Tin Dioxide Gas Sensing Materials: Mechanistic Insights and the Role of Gold Dispersion David Deglera,b, Sven Ranka,b,†, Sabrina Müllerc, Hudson W. Pereira de Carvalhoc,d, Jan-Dierk Grunwaldtc, Udo Weimara,b and Nicolae Barsana,b,* a
Institute of Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 15, 72076 Tübingen, Germany b
Centre for Light-Matter Interaction, Sensors & Analytics (LISA+), University of Tübingen, Auf der Morgenstelle 15, D-72076 Tübingen, Germany
c
Karlsruhe Institute of Technology, Institute for Chemical Technology and Polymer Chemistry, Engesserstr. 20, 76131 Karlsruhe, Germany
d
Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, P. O. Box 96, 13400-970, Piracicaba, SP, Brazil
KEYWORDS: Gas Sensor, SnO2, Gold, Operando Spectroscopy, Oxygen Activation, Spillover ABSTRACT: This work focuses on two aspects of gold-loaded tin dioxide gas sensing materials: The influence of the size and dispersion of the gold on the sensing effect and the investigation on the mechanism at the origin of the improved gas sensing performance. For this purpose, a set of selected and well-characterized gold loaded tin dioxide materials were examined. The results show that the beneficial effect of gold on the CO sensing performance is observed for nano-sized as well as for micron-sized gold entities, i.e. the effect is related to Au itself. Nevertheless, the response is strongly enhanced with increasing gold dispersion. Deeper insights into the mechanism of the sensitization, obtained by state-of-the-art operando spectroscopic techniques, indicated that oxygen is adsorbed on gold and transferred to the tin dioxide surface. There, it is bound as a negatively charged, ionic species, which gives additional sites for the interaction with target gases, i.e. enhances the gas sensing performance. These results strongly support the previously proposed oxygen spillover mechanism for gold-loaded tin dioxide.
Metal oxides loaded with gold (Au) have fascinating properties for oxidation catalysis1–6 or gas sensing7–12. The gas sensor performance of semiconducting metal oxides used for sensing reducing gases are not only strongly increased by the presence of Au nanoparticles7–11, but also by macroscopic Au, like Au electrodes13,14 or Au particles in the µm range15. Although the Au loaded materials were very thoroughly investigated and in all cases Au was found in its metallic form7,8,10,11,16, mechanistic insights into the sensitization of metallic Au, as such, are still a matter of discussion. The proposed models assume a spillover of target gases (H2 or CO)10,17 or the spillover of either neutral8 or ionized11 molecular oxygen from the Au to the metal oxide surface. On the basis of operando Xray Absorption Spectroscopy (XAS) and combined DC resistance and work function measurements M. Hübner et al. proposed an oxygen spill over mechanism for Auloaded SnO2.8 Because for CO oxidation catalysts a strong influence of the Au particle size on the CO oxidation performance is reported, it cannot be excluded that the size of the Au loadings affect the Au-related sensitization
of the gas sensors.6,18 Consequently, the following questions arise: • Do the magnitude or mechanism of the Au-related sensitization of SnO2 depend on the size of the Au loading? • Is there solely a spillover of oxygen or is there also an activation of the target gases? Inspired by the previous study by M. Hübner et al. 8 and in order to answer these questions this work investigates the sensing performance of three SnO2-based materials with different Au particles sizes and loadings and the mechanisms involved therein. The used methods are state of the art operando spectroscopic techniques, namely XAS and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), complemented by a systematic electrical sensing performance study
Experimental Material preparation and sensor fabrication A set of one undoped and three differently Au-doped SnO2 materials was synthesized. The SnO2 base material was synthesized from SnCl4 (Merck, purified by distilla-
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tion) in an aqueous sol-gel process as described elsewhere.19 The gold loading was achieved by three different methods: •
Micron-sized metallic Au particles (Gold Powder, Ferro Coperantion) were physically mixed with the undoped SnO2 powder; in the following this material is referred as µm-mix.
•
Another material was made by adding an aqueous solution of AuCl3 to a suspension of previously calcined (1000 °C, 8 h) SnO2, the mixture was stirred for 48 h at room temperature and dried. Subsequently, the obtained powder was thermally treated at 450 °C (1 h); this material is referred as powder-impregnated (PI).
•
The third material was synthesized by depositing colloidal Au onto the surface of calcined SnO2 as described elsewhere.20,21 The mixture was stirred for 10 min, filtered, dried and thermally treated at 400 °C (1 h); this material is referred as colloidal (Col).
Sensors were made by screen-printing a propandiol-based paste onto Al2O3 substrates equipped with platinum electrodes (front) and heater (back).22 The printed sensors were dried at 70 °C overnight and finally annealed using a four-zone belt oven (300 °C, 400 °C, 500 °C and 400 °C). All materials are listed in Table 1 with additional information on the metal loadings and the obtained Au particle sizes.
Characterization UV/vis DRS UV/vis spectra were recorded with a Perkin-Elmer LAMBDA650 in diffuse reflectance geometry (UV/vis DRS) using a commercial mirror optics (Harrick, Praying Mantis). All spectra were measured with a spectral resolution of 1 nm, an integration time of 0.4 s, a scan speed of 141 nm/min and a slit width of 2 nm. The sample spectra were recorded from thick film gas sensors at room temperature using a homemade sample holder. The sample spectra were referenced to a BaSO4 layer screen printed on a sensor substrate.
Electron microscopy Scanning Electron Microscopy (SEM) was done with a HITACHI SU8030 cold field emission scanning electron microscope equipped with a Bruker Quantum EDS (XFlash 6|60 detector) for Energy Dispersive X-Ray spectroscopy (EDX). The microscope was operated with an acceleration voltage of 5 or 15 keV, the latter one for EDX measurements, and a probe current of 8 or 10 µA, respectively. All micrographs made by SEM were recorded from sensing layers of thick film gas sensors and measured in the secondary electron detection mode. Small amounts of the powdered Col and PI samples were suspended in ethanol, ultrasonically dispersed, drop casted onto carbon coated Cu-grids and dried. A FEI Titan 80300 aberration corrected electron microscope operated at 300 keV at the Karlsruhe Nano Micro Facility (KNMF) was
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used to examine the prepared samples for the determination of the particle size and distribution on the support material. Scanning Transmission Electron Microscopy (STEM) images were acquired by a Fischione model 3000 HAADF-STEM detector. To verify the composition of the samples EDX spectra were acquired by an EDAX SUTW EDX detector.
ICP-OES Optical Emission Spectrometry with Inductively Coupled Plasma (ICP-OES) was used to evaluate the metal content of the PI sample to reaffirm the influence of the metal content and particle sizes on the observed sensor performance. The material was dissolved in an acidic solution during a temperature-pressure-program using a Berghof DAB-2 vessel at the Institute for Applied Materials - Applied Materials Physics (IAM-AWP) at the Karlsruhe Institute of Technology (KIT). The elements were subsequently determined by ICP-OES (triple determination).
X-ray absorption spectroscopy The sample materials were investigated in form of pellets and under operando conditions by High Energy Resolution X-ray Absorption Spectroscopy (HERFD-XAS) to increase the sensitivity towards changes at the Au L3-edge at the insertion device beamline ID26 at the European Synchrotron Radiation Facility (Grenoble, France). The measurements were carried out at Au-L3 edge; both Extended X-ray Absorption Fine Structure (EXAFS) and Xray Absorption Near Edge Structure (XANES) spectral regions were recorded. For that a 200 x 200 µm² monochromatic (Si311) focused beam was employed. X-ray fluorescent photons were dispersed by four Ge(555) crystal analyzers and the Lβ1,3 line photon yield was counted by an avalanche photodiode. For the operando-XAS investigations a homemade measurement cell was used.5 The cell allowed the operation under sensing conditions (250 ppm CO either in air or N2) at a sensor operation temperature of 250 °C. Reference HERFD-XANES spectra were recorded at sensor operation temperature in pure N2 and air. XAS data analysis steps such as averaging, normalization and background subtraction were performed with Athena of the IFEFFIT software package.23 HERFDXANES spectra of an Au19-cluster in CO and O2 atmospheres have been simulated using FEFF9 code. In the calculations only the linear atop position of the two molecules was considered. All Au atoms were treated equally and full coverage was expected. Infrared spectroscopy Operando DRIFTS was conducted using a Bruker Vertex70v FT-IR spectrometer equipped with an external high-performance globar and midband MCT detector. Spectra were recorded with a spectral resolution of 1 cm-1 and each spectrum was averaged from 512 scans. The gas sensor was placed in a homemade operando cell, fixed in a mirror optic (Harrick, Praying Mantis), connected to a digital multimeter a power supply and heated to 250 °C during the experiment. Gases were dosed as described below. Difference spectra were calculated as described elsewhere.24,25
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Figure 1. STEM images of PI (a) and Col (c) with the corresponding diagrams of the relative intensities of the found particle sizes, (b) and (d), respectively.
DC-resistance measurements The sensor performance was examined by DC-resistance measurements in different gas atmospheres. Gases were dosed using a home-made gas mixing station equipped with mass flow controllers. The sensor resistance was recorded using a scanning multimeter (Keithley 199). The CO sensing performance was evaluated in the range of 5 to 500 ppm in dry air at a constant flow of 150 mL/min. The effect of low oxygen concentrations (100 to 1600 ppm) dosed in dry nitrogen was measured at a flow rate of 400 mL/min using an ultra-tight system with welded stainless steel tubing. The actual oxygen concentration was monitored using an oxygen sensor (Zirox SGM 400); in dry nitrogen the oxygen background was below 1 ppm. During all experiments the sensors were heated to 250 °C. The residual water concentration in dry conditions was found to be lower than 10 ppm.
Results & Discussion The two 0.2 %wt-loaded samples (PI and Col) show a changed color due to the Au loading, which is caused by the localized Surface Plasmon Resonance (SPR) of nanosized Au clusters.26 The UV/vis-DR spectra (see Supporting Information Figure S1) show SPR absorption bands at
554 and 549 nm for PI and Col, respectively. The position of the SPR band correlates with the Au cluster size, i.e. larger clusters show a SPR band at higher wavelength.3 Accordingly, the PI and Col sample show comparable small Au clusters. No SPR band was found for the µmsized Au particles. A more detailed picture of the size and the distribution of the Au particles on the surface of the SnO2 grains has been obtained by electron microscopy (see Figure 1 and Figure 2). Whereas many small Au clusters were clearly visible in the Col sample (Figure 1c) there were fewer particles on the SnO2 surface of the PI sample (Figure 1a). An additionally conducted determination of the actual metal content revealed a loading of only 0.08 wt%-Au for the PI sample (see Table 1). The lower coverage with Au nanoparticles for the PI compared to the Col sample may be assigned to the significantly smaller metal loading, as found by the element analysis. The micrograph of µm-mix material (Figure 2a) reveals rather large Au particles with an average size of 1.36±0.29 µm (see Supporting Information Table S1 and Figure S2 a-c) incorporated in the sensing layer, which consists of SnO2 grains of a size less than a tenth of the Au particles. The Au particles were identified by an EDX line scan (Figure 2b), which clearly shows that Au is only present in form of the µm-sized particles.
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Table 1. Summary of the synthesis and characteristics of all materials. Thermal treatments Material
undoped
Method
--
st
nd
1
2
Au concentration Nominal
ICP-OES
dAu
λSPR
Oxidation * state
[°C]; [h]
[°C]; [h]
[%wt]
[%wt]
[nm]
[nm]
1000; 8
--
0.0
n/a
--
--
--
--
metallic
#
554
metallic
#
549
metallic
µm-mix
Dry mixing with Au particles
1000; 8
--
5.0
n/a
1.4·10
PI
Impregnation with AuCl3-solution
1000; 8
450; 1
0.2
0.08
13
Col
Adsorption of Au colloids
1000; 8
450; 1
0.2
n/a
7
+
3+
by SEM/EDX; # by STEM; * by XANES
It should be noted that the Au particles show a tendency to form agglomerates of several particles (see Supporting Information Figure S2 a-c). Information on the oxidation state of the Au-loading was obtained by analyzing a XANES spectrum of each sample. The XANES of the µm sized and PI-sample were recorded at room temperature whereas the one of the Col-sample was collected during operando-measurements in air at 250°C and is included for comparison. All XANES spectra were normalized by the edge step determined from the difference between the monotonous decrease of the absorption coefficient before and after the absorption edge. The data of an Au-foil is included as reference. The EXAFS data (not shown), complementarly to the HERFD-XANES discussed below, showed that in all samples the Au particles were fully reduced. This information can be derived from the white line intensity of the XANES spectra in the Supporting Information (Figure S3). An overview of the material preparation and characterization is given in Table 1.
Figure 2. Typical electron micrograph of µm-mix measured from a gas sensing layer (a) and a corresponding magnified image section with an EDX line scan (b) for oxygen (O Kα1, red line), tin (Sn Lα1, blue line) and gold (Au Mα1, green line).
Figure 3 presents the calibration curves of CO in dry air for undoped SnO2 and all Au-loaded samples. The sensor signals of all four materials show a power law dependency on the CO concentration, which is typical for metal oxidebased gas sensors.27,28 Compared to the undoped material, all Au-loaded materials show a strongly enhanced sensor signal, which – in case of the colloidal gold loading – is 10 times higher. It is a rather interesting finding that not only the nano-sized Au loading has such an enhancing effect, but also the µm-sized Au particles – albeit at a much higher Au concentration – are beneficial for CO sensing. This observation suggests that the improved gas sensing performance is related to the general presence of the noble metal. Comparing the two materials with nanosized Au particles the colloidal material shows the highest sensor performance.
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Figure 3. Sensor signals for 5 to 500 ppm CO dosed in dry air: undoped SnO2 (a) µm-mix (b), PI (c), and Col (d). During the measurement the sensors were heated to 250 °C.
Figure 4. Effect of different CO concentration dosed in dry synthetic air (top) and different O2 concentration dosed in dry N2 (bottom) for undoped SnO2 (a) µm-mix (b), PI (c), and Col (d). During the measurements the sensors were heated to 250 °C.
Table 2. Fitted n-values for all four materials. The n-value corresponds to the slope of the linear fits in Figure 4. n-values Material CO in dry air
O2 in dry N2
undoped
-0.420
0.132
µm-mix
-0.509
0.253
PI
-0.461
0.355
Col
-0.491
0.460
Similar to the sensor signals, also the resistance of the sensor shows a power law dependency that can be linear-
ized by a double logarithmic plot of the gasconcentration and sensor resistance (Figure 4) The comparison of slopes of the linearized plots, which are the values of the exponent in the power law, also reffered as n-values, allows identifying changes in the reception of the sensor, i.e. the surface chemistry, and the transduction function, i.e. the translation of the chemical changes into an electrical signal.27–29 The n-values for CO in dry air (see Table 2) are, all in all, at a comparable level for all materials. Thus, it is unlikely that the enhanced sensing performance is related to a strongly altered interaction with CO. A rather significant difference is found when comparing the effect of O2 dosed in dry N2 (Figure 4 bottom). The n-values for O2 of the Au-loaded materials are higher than for the undoped sample, i.e. the Au samples show a higher capability to ionosorb oxygen. For all materials the difference in the nvalues for O2 correlates with the improved sensing performance (Figure 3). Therefore, and in agreement with previous measurements8, the enhanced ionosoprtion of oxygen can be identified as key factor, which improves the gas sensing performance of Au-doped SnO2. A change of the transduction can be excluded since the resistance values are all the time above the resistance level found in a pure nitrogen flow, i.e. the transduction follows a depletion layer controlled mechanism for all experiments.30,31 The above discussed findings suggest that the enhanced sensing performance of Au is based on the dispersion and thus the increased Au-SnO2 interface. The material prepared by colloidal deposition shows the smallest Au clusters, thus the operando spectroscopic investigation was done only for the colloidal sample. The impact of oxygen and CO on the oxidation state and electron density of the Au clusters was studied by operando HERFD-XANES (Figure 5) and compared with theoretical calculations (Figure 6). High energy fluorescence detected XAS spectra were taken to increase the sensitivity. Nevertheless, only small but not negligible, and reversible shifts in the edge position are observed for the different atmospheres. In the presence of 250 ppm CO (Figure 5 a&b) the Au L3 edge is shifted towards higher energies compared to the spectra recorded under the corresponding carrier gas, dry air (Figure 5d) or dry N2 (Figure 5e). On the opposite, the addition of O2 leads to a shift to lower energies (Figure 5f). For comparison and a better understanding of the results, the XANES spectra at the Au L3 edge were calculated for an Au19-cluster with adsorbed CO or O2 using FEFF9 code.18 Figure 6 shows the theoretically simulated HERFD-XANES spectra. The tendencies observed experimentally match the simulation: The adsorption of CO or O2 leads to a decrease of the edge energy, whereby the edge during the adsorption of CO was shifted to higher energies compared to the oxygen adsorption. The more enhanced shift for adsorbed CO can be explained by electron back-donation into anti-bonding CO-orbitals.18 However, the experimental shifts were not as significant as the theoretically calculated ones, indicating a weak interaction of CO, a comparatively stronger interaction of O2 with the Au surface and the small number of sites effected due to the particle size of Au above 5 nm.
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Figure 5. Operando XANES spectra of the Col sample recorded at 250 °C during the exposure of 250 ppm CO in air (a, d) and nitrogen (b, e). The spectra compared to the corresponding spectra recorded under the pure carrier gas. Additional the XANES spectra under air and nitrogen (36 ppm residual O2) are shown (c, f). Figures d to f show a magnification of the absorption edge of the corresponding figures a to c. The corresponding measurements of the sensor resistance and the oxygen concentration are shown in the Supporting Information (Figure S4).
This fact encourages future attempts to prepare even smaller Au clusters. N. Guo et al.32 studied the effect of CO adsorption on the white line position in XANES.They found that the surface coverage of noble metal nanoparticles with CO affects the XANES spectra resulting in small changes in the shape and energy of the white line. However, operando DRIFTS do not show the presence of stable Au-CO species at 250 °C for 250 ppm CO dosed in dry air (Supporting Information Figure S5a) or dry N2 Supporting Information Figure S5b). Taking the operando DRIFT spectra into account it is unlikely that the cause the shift of the white line is a high coverage with adsorbed CO of the Au clusters. J. A. van Bokoven et al.18 found that the adsorption of O2 on small Au clusters (d < 3 nm) leads to a transfer of charge from the d-band of the Au to the 2π*-orbital of oxygen molecules, which causes a decrease of the white line energy. This observation is in line with the much weaker shift of the white line observed when comparing the XANES spectra recorded in air and nitrogen (Figure 5c). This less pronounced shift can thus be explained by the larger average size of 7 nm of the Au particles on the SnO2 support. In case of the CO exposure in N2 the O2 background concentration was further de-
creased from 36 ppm in pure nitrogen to 1 ppm in the presence of CO (see Supporting Information S4), thus the shift of the white line could be caused by a decreased oxygen coverage. A changing oxygen coverage on the Au would also explain the stronger shift of the white line
Figure 6. FEFF9-calculated Au L3 HERFD-XANES spectra of an Au19-cluster with adsorbed CO or O2.
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when 250 ppm CO are dosed in air, since the initial O2 coverage should be higher and thus allow higher changes.
bands may correspond to additional (atomic) oxygen species (overtones of the Sn-O-Sn vibrations35) on the SnO2 surface, or to uncharged molecular oxygen (O-O stretch vibration36,37) species adsorbed to the surface of the Au clusters. A specific assignment of the oxygenrelated bands in the DRIFT spectra would be speculative,however, the occurrence of new oxygen species has been demonstrated. Considering the experimental and theoretical findings, the following statements can be made: The XANES spectra show a weak interaction of the Au clusters with either O2, CO or both. Taking into account the DRIFT spectra a strong adsorption of CO is unlikely, while an increased oxygen adsorption on the Au-doped SnO2 is found. Based on the resistance measurements, the increased adsorption of oxygen involves the trapping of electrons from the conduction band of SnO2 forming ionosorbed oxygen species on SnO2 and increasing the reaction partners for CO. In summary, the improved CO sensing performance of Au-doped SnO2 can be explained by the following mechanism: Oxygen molecules are adsorbed on the Au surface (i) and spilled over to SnO2, where dissociated oxygen species are ionized with electrons from SnO2 (ii). In the presence of CO the ionosorbed oxygen reacts with CO forming CO2 and injecting the electrons back into the conduction band of SnO2 (iii). As the concentration of ionosorbed oxygen species is larger on the Au-doped materials, a larger number of electrons are released to the conduction band under CO exposure causing a higher sensor signal. The proposed model for the described process is depicted in Figure 8.
Figure 7. Operando DRIFT spectra of the Col sample recorded at 250 °C during the exposure of 250 ppm CO dosed in air (a) and nitrogen (b). The spectra are referenced to the corresponding carrier gas, air or nitrogen. Additionally the air
supports the proposed spillover sensitization of Au-doped SnO2 sensing materials.8 spectrum referenced to pure nitrogen is shown in c. A magnified figure of the carbonyl region is found in the Supporting Information (Figure S5).The corresponding measurements of the sensor resistance and the oxygen concentration are shown in the Supporting Information (Figure S4).
The involvement of additional oxygen species is corroborated by the operando DRIFT spectra (Figure 7c): A series of bands, additional to the ones assigned to Sn-O lattice overtones found for undoped SnO2 (1362, 1334 ,1271, 1159 and 1059 cm-1)19, is observed at 1395, 1442, 1488, 1532 and 1597 cm-1. Since these bands decrease in the presence of CO and increase when the oxygen concentration is increased, the identification as formiate, carbonate or carboxylate species, as reported for the low temperature oxidation of CO33,34, is unlikely. It is important to note that on the corresponding undoped SnO2 material such species were not found.19 Moreover, the bands are not in the typical region of Sn-OH deformation vibrations.35 Thus they may be assigned to oxygen species formed on the surface on the Au-doped SnO2. Alternatively, the
Figure 8. Proposed model for the oxygen activation and improved sensor performance on Au loaded SnO2, involving three fundamental steps: (i) Molecular oxygen is adsorbed on the gold surface, (ii) During the spillover process from the Au to the SnO2 surface oxygen dissociates and is ionized by electrons from the conduction band of SnO2 and (iii) CO reacts with ionosorbed oxygen on the SnO2 surface, releasing electrons back to the conduction band of SnO2
Conclusions The analysis of the electrical measurements of undoped and Au-loaded SnO2 materials revealed: • The sensitization effect of Au is not limited to nanosized Au clusters
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• The Au-related sensitization increases with the dispersion of the Au loading on the sensing material as this increases the boundary between Au and SnO2
Notes
• Changes in the oxygen chemistry cause the sensitization rather than a changed reaction of CO with the surface
ACKNOWLEDGMENT
However, it is desirable to apply a uniform introduction method for Au in the future, which excludes a possible influence of the introduction method on the obtained results. The operando spectroscopic XAS and DRIFTS studies of the colloidal sample provided a deeper insight in the changes of the surface chemistry: • Since no indications for charged oxygen species are found, it can be concluded that oxygen is adsorbed as a molecular and uncharged species on the Au clusters/particles • Subsequently oxygen is spilled over to the SnO2 surface, where - after dissociation - oxygen is bound as an ionized atomic species, trapping electrons from the conduction band of SnO2 • While an improved oxygen adsorption is supported by the experimental findings, an increased adsorption and spillover of CO is not corroborated by the experimental results In conclusion, this work highlights the outstanding difference of the sensitization mechanism of Au compared to to the electrical and/or chemical sensization by Pd or Pt.5,38–42 The presented results support the hypothesis that the sensitization of Au-loaded SnO2 gas sensors is related to an oxygen spillover. Furthermore, it demonstrates that the rational selection and good characterization of materials as well as the use of appropriate, i.e. in-situ or operando, investigation techniques provides a solid basis for the understanding of the gas sensing process.
ASSOCIATED CONTENT
The authors declare no competing financial interest.
Dr. Di Wang of INT-EMSL (KIT) and Dr. Sina Baier (KIT) are acknowledged for performing the electron microscopy analyses of the different sensing materials and Hilmar Adler of Institute of Physical and Theoretical Chemistry (University of Tübingen) for performing the electron microscopy analysis of the sample with µm-sized Au. We further thank Martin Reichardt for preparing first colloidal gold particles on SnO2 during his diploma thesis. The authors also thank the ESRF (Grenoble, F) for beamtime allocation at the beamline ID26 and financial support in frame of proposal MA 1974 and the beamline scientists, namely Dr. Pieter Glatzel and Dr. Erik Gallo, and all staff of ID26 for the extensive support and inspiring discussions during the beamtime.
REFERENCES (1)
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Supporting Information. Additional spectroscopic, microscopic and electrical information is available free of charge on the ACS Publications website at DOI: (7)
AUTHOR INFORMATION Corresponding Author * Dr. Nicolae Barsan Institute of Physical and Theoretical Chemistry University of Tübingen Auf der Morgenstelle 15, 72076 Tübingen, Germany E-mail:
[email protected] Tel: +49 7071 29 78761
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Present Addresses †Sven Rank, Audi AG, N/EA-65, Postfach 1144, 74148 Neckarsulm
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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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